Re: [Xen-devel] [PATCH RFC 2/3] x86/hvm/rtc: Inject RTC periodic interupts from the vpt code.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.14 at 18:32, Tim Deegan <tim@xxxxxxx> wrote:
> Let the vpt code drive the RTC's timer interrupts directly, as it does
> for other periodic time sources, and fix up the register state in a
> vpt callback when the interrupt is injected.
> 
> This fixes a hang seen on Windows 2003 in no-missed-ticks mode, where
> when a tick was pending, the early callback from the VPT code would
> always set REG_C.PF on every VMENTER; meanwhile the guest was in its
> interrupt handler reading REG_C in a loop and waiting to see it clear.
> 
> One drawback is that a guest that attempts to suppress RTC periodic
> interrupts by failing to read REG_C will receive up to 10 spurious
> interrupts, even in 'strict' mode.  However:
>  - since all previous RTC models have had this property (including
>    the current one, since 'no-ack' mode is hard-coded on) we're
>    pretty sure that all guests can handle this; and
>  - we're already playing some other interesting games with this
>    interrupt in the vpt code.
> 
> One other corner case: a guest that enables the PF timer interrupt,
> masks the interupt in the APIC and then polls REG_C looking for PF
> will not see PF getting set.  The more likely case of enabling the
> timers and masking the interrupt with REG_B.PIE is already handled
> correctly.
> 
> Signed-off-by: Tim Deegan <tim@xxxxxxx>

Looks plausible too, and the revert it is effectively doing is
obviously acceptable now with patch 1 in place. Curious to know
what's still broken with it...

Jan


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